The invention is directed to a quick-action cylinder with a safety device to prevent blocking including a mechanical lock for a center draw-in nipple in a center bore in a housing. In the locked position, one or more locking members are urged into contact with an exterior periphery of the draw-in nipple by a force of a spring assembly which is arranged in the housing in one or several spring compartments. The unlocked position of the draw-in nipple is achieved by applying pressure to a locking piston which operates to oppose the force of the spring assembly, wherein at least one relief bore which extends to a clearance space having a lower pressure is arranged in the spring compartment.
Quick-action cylinders of the aforedescribed type have been described in a number of publications. For example, reference is made to DE 198 34 040 A1 or DE 198 34 040 C2 where the basic functionality is described. The disclosure of the aforedescribed patent applications is included in the present disclosure by reference in its entirety.
Quick-action cylinders of the aforedescribed type operate with a mechanical lock for the draw-in nipple secured on a workpiece palette. It is known to lock a draw-in nipple which is pulled into the housing of the quick-action cylinder by a spring force. The draw-in nipple is locked using locking members which contact the periphery of the draw-in nipple and hold the draw-in nipple in the locked position in the housing of the quick-action cylinder.
The locked position is achieved by springs which are typically implemented as disk springs and are supported at one end on the bottom face of the quick-action cylinder and at the other end by a locking piston operated by a hydraulic fluid or compressed air.
Instead of the aforedescribed disk springs, other types of energy stores known in the art can also be used. It is a characteristic feature of this type of quick-action cylinders that the locking members are locked with the draw-in nipple by way of a spring force and are unlocked by applying pressure from a hydraulic fluid or compressed air.
For the sake of clarity of the following description, only the application of a pressure medium in the form of a hydraulic fluid is described, although the invention can also be used with other pressure media, for example glycol, air and the like.
For unlocking, a pressure medium is introduced between the housing and the movable sealed locking piston, wherein the pressure medium moves the locking piston against the force of the springs so as to release the locking members and disengage them from the draw-in nipple.
For this type of quick-action cylinders, it is also necessary to provide a dependable seal between the center bore, in which the draw-in nipple is inserted, and the radially outwardly disposed spring compartments which receive the spring assembly.
If this seal is defective, then there is a risk that water entering the center bore may reach the adjacent spring compartment, potentially damaging the spring assembly through corrosion. The spring compartment is therefore typically sealed to keep out contaminants. The spring compartment is hence hermetically sealed against the other housing components.
In has been observed in several applications that the seal between the locking piston and the housing became defective or was already damaged during assembly of the quick-action cylinder. As a result, when a pressure medium was introduced into the cylinder space, this medium was then able to reach the (sealed) spring compartment via the defective seal. Such operating condition rendered the quick-action cylinder inoperative due to self-blocking.
When the hydraulic fluid reached the spring compartment due to inadequate sealing between the cylinder and the housing of the quick-action cylinder, then the spring compartment filled up with hydraulic fluid, thus blocking the locking piston, because the springs act in opposition to the locking piston. The cylinder compartment filled with the hydraulic fluid acted on one side of the locking piston, whereas the same hydraulic fluid acted on the locking piston from the other side (with a significantly larger area). This blocked the locking piston, because the spring force of the spring assembly was applied in addition to the force exerted on the locking piston in addition by the leaking hydraulic fluid from below.
The quick-action cylinder could the no longer be unlocked, eliminating any possibility to access the quick-action cylinder from the outside for unlocking the cylinder, because the top side of the quick-action cylinder was typically covered by the screwed-on change palette and the bottom side of the quick-action cylinder was mounted in a recessed receiving opening in the support table. The quick-action cylinder was therefore no longer accessible from the outside in the operating state, and the aforementioned blocking caused by defective seals could no longer be remedied.
The only possible solution was then to increase the hydraulic pressure on the locking piston in the cylinder to a point where the cover was blown off upwardly, which was associated with a substantial risk for injury.
DE 100 06 847 C2 discloses a simple discharge bore from the spring compartment to the center bore to discharge contaminated water entering the spring compartment through the discharge bore to the center bore. For this purpose, the discharge bore was connected to the bottom section of the spring compartment for completely removing the water.
However, it has been observed that this type of water removal from the spring compartment is not adequate for preventing contaminated water, in particular water contaminated with chips, from entering the spring compartment.
Water is typically removed from the center bore by blowing the water out with a compressed air gun. However, water contaminated with chips is thereby pushed into the discharge bore and thereby also into the spring compartment which can seriously damage the spring assembly.
In addition, when the draw-in bolt enters the center bore, an overpressure is produced which presses the water, which is contaminated with chips and deposited on the bottom of the center bore, through the discharge bore into the spring compartment.
An additional air supply channel must therefore be provided on the top side of the spring compartment. The contaminated spring compartment must then be cleaned by blowing in air, returning the water contaminated with chips to the center bore.
The discharge bore which transports unpressurized water can also be obstructed by inflowing contaminants. Such an arrangement is therefore not suited to prevent blocking of the locking device if contaminated water enters the spring compartment.